Organotitanium compounds and, in particular, titanium alkoxides are known as catalysts for esterification processes. During the esterification, these compounds are frequently converted to polymeric compounds of titanium which result in a hazy product. The presence of a haze is a particular disadvantage in polyesters which have a high viscosity and/or high melting point and are therefore difficult to filter. Furthermore, many organotitanium compounds which are effective catalysts in the manufacture of polyesters such as polyethylene terephthalate are known to produce unacceptable yellowing in the final polymer. Tetra(n-butyl)titanate (TNBT) is a widely used catalyst for the preparation of polybutylene terephthalate (PBT). TNBT is susceptible to hydrolysis which may lead to the formation of small amounts of solid hydrolysis products which can reduce the effectiveness of filtration equipment used in the manufacturing process.
In GB-A-2314081, an esterification catalyst comprising the reaction product of an alkoxide or condensed alkoxide of titanium or zirconium, an alcohol containing at least two hydroxyl groups, a 2-hydroxy carboxylic acid and a base. In that patent, the preferred amount of base for monobasic 2-hydroxy acids such as lactic acid, is in the range 0.8 to 1.2 mole per mole of 2-hydroxy acid. In the case of citric acid (a tribasic acid), the preferred amount is in the range 1 to 3 moles base per mole of 2-hydroxy acid. We have found that when a catalyst composition of the type described in GB-A-2314081 is made using 1,4-butane diol as the dihydric alcohol and an inorganic base is present in the preferred concentration range, the 1,4-butane diol forms a complex with the base leading to formation of a gelled product which is not suitable for use as a catalyst.